1. Field of the Invention
This invention relates to a scanning read-out apparatus for scanning a read-out objective with a light beam by use of a light scanning means such as a galvanometer mirror, detecting the information recorded on the read-out objective, and generating a read-out signal, which is provided with a means for correcting the read-out signal with respect to a fluctuation in scanning speed in order to eliminate adverse effects of the fluctuation in scanning speed from the read-out signal. This invention also relates to a method of adjusting a correction coefficient used for eliminating the noise component caused by the fluctuation in scanning speed.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in the pattern of the stored image. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as a photographic light-sensitive material or on a display device such as a cathode ray tube (CRT).
FIG. 4 is a perspective view showing the conventional read-out apparatus used in the aforesaid radiation image recording and reproducing system for scanning a stimulable phosphor sheet carrying a radiation image stored therein by stimulating rays such as a laser beam which cause the sheet to emit light in proportion to the stored radiation energy, and photoelectrically detecting the emitted light.
In the apparatus of FIG. 4, stimulating rays 2 are emitted by a stimulating ray source 1, and the beam diameter of the stimulating rays 2 is precisely adjusted by a beam expander 3. The stimulating rays 2 are then deflected by a scanning means 4 formed of a galvanometer mirror or the like, and are made to impinge upon the stimulable phosphor sheet 10 by a plane reflection mirror 5. Between the scanning means 4 and the plane reflection mirror 5 is positioned an f.theta. lens 6 for maintaining the beam diameter of the stimulating rays 2 uniform during the equal-speed scanning of the stimulating rays 2 on the stimulable phosphor sheet 10. While the stimulating rays 2 impinge upon the stimulable phosphor sheet 10, the sheet 10 is moved in the direction as indicated by the arrow A (i.e. sub-scanning direction ) and, consequently, the whole area of the sheet 10 is exposed to and scanned by the stimulating rays 2. Upon exposure to the stimulating rays 2, the stimulable phosphor sheet 10 emits light in proportion to the radiation energy stored therein, and the light emitted enters a light guide member 8. The light entering the light guide member 8 is transmitted through total reflection inside of the light guide member 8, and received by a photodetector 9 such as a photomultiplier. The light guide member 8 and the photodetector 9 constitute a photoelectric read-out means 7. The light detected by the photodetector 9 is converted into an electric image signal, which is passed through an amplifier 11, an A/D converter 12 and a signal processing circuit 13, and then used to reproduce a visible image on a recording material such as a photographic film or a display device such as a CRT. The light guide member 8 may be of a shape and a material as disclosed in U.S. Pat. No. 4,346,295.
However, in the apparatus of the type scanning a light beam by use of scanning means such as a galvanometer mirror, the scanning speed of the light beam along a scanning line 14 is often caused to fluctuate by a fluctuation in movement speed of the scanning means, for example, by a fluctuation in the swinging speed of the galvanometer mirror. When the scanning speed fluctuates, the read-out image signal obtained by the scanning contains adverse effects of the fluctuation in scanning speed, and it becomes not always possible to conduct the scanning read-out with high accuracy.
One approach to elimination of the problem caused by the fluctuation in scanning speed is to eliminate the scanning speed fluctuation components of the scanning means by controlling the movement speed of the scanning means which causes the fluctuation.
In this method, of the scanning speed fluctuation components, the low-frequency component can be eliminated by controlling the scanning means. However, it is not always possible to eliminate the high-frequency component by controlling the scanning means.